It's hard to keep perfect posture all day at a desk. Neck rolls will help counter the negative effects of poor posture.
Cat to Cow
This is a subtle back stretch. You might not even feel anything at first, but it will definitely loosen you up.
Seated Forward Bend
This is a bit more difficult for beginners, but it's worth the work. This works your lower back, shoulders, and chest.
Put your wrists on the desk and point your fingers towards your body. Put some weight on it, but back off the moment you feel pain.
Put on foot on the desk so that your knee also rests on the desk. Lean into the stretch gently.
Eagle arms loosens your back and chest muscles. It also helps release tension from your forearms and wrists.
Sitting in a chair all day can make your back incredibly stiff. The seated twist wrings the stiffness out. Chances are that you'll hear a couple good pops.
Sitting on a balance ball helps strengthen your core. You don't even have to do anything. Just stay upright.